As a ranger every time I enter and leave water my land pet respawns with full health and his F2 ability recharged. So basically, if I am fighting and my pet is about to die (or I have used an F2 ability with a long cool down), all I have to do is jump in water which removes the pet, then emerge from water and the pet has full health and his F2 ability is available immediately. I assume this is a bug since it can be used to exploit the pet mechanic and basically give you an unkillable pet with an unlimited F2 ability.